The hydrogen bonds in DNA Watson–Crick base pairs have long been considered predominantly electrostatic phenomena. In this chapter, we show with state-of-the-art calculations that this is not true and that electrostatic interactions and covalent contributions in these hydrogen bonds are in fact of the same order of magnitude. We discuss the steric and electronic effects of the backbone and the influence of the molecular environment on hydrogen bonding in Watson–Crick pairs. Furthermore, we address the question of how far these hydrogen bonds can be considered resonance-assisted hydrogen bonds (RAHB). Finally, the implications of our findings for our understanding of the highly accurate process of DNA replication are discussed.